Viscous mean drift forces on moored semi-submersibles
- Delft Univ. of Technology (Netherlands). Ship Hydromechanics Lab.
Theoretical predictions using 3-D potential theory for the mean drift forces on moored floating structures like semi-submersibles and tension leg platforms show discrepancies in both regular and irregular waves when compared with results of model tests. Such divergence is further pronounced in the low frequency range (storm condition) where diffraction effects are less for such structures and is thus believed to be induced by viscous effects. The viscous drag term of the Morison equation in combination with the linear (Airy) theory up to the instantaneous wave elevation has been considered as the basis in order to develop the theory in a waves-only field as well as in a wave-current coexisting flow field. Experimental investigations have been carried out with fixed vertical surface piercing model cylinders and a submerged pontoon in regular waves. Test results validate the theory with a further indication that the mean forces on completely submerged bodies are not influenced significantly by viscous effects in a waves-only field. Finally, experiments have been carried on for a complete semisubmersible model in regular waves in fixed and free floating conditions at zero speed and with forward velocity. Forces have been measured for the complete model as well as for some individual columns in order to gain an insight into any interference effect on the viscous mean drift forces. The theory, incorporating a relative horizontal velocity and relative surface elevation concept, enhances the theoretical predictions for the horizontal mean drift forces when experimentally obtained values of the mean drag coefficients for the cylinders and the submerged pontoon are properly applied for the complete semi-submersible.
- OSTI ID:
- 260413
- Report Number(s):
- CONF-950604-; ISBN 1-880653-19-2; TRN: IM9632%%72
- Resource Relation:
- Conference: 5. international conference on offshore and polar engineering, The Hague (Netherlands), 11-15 Jun 1995; Other Information: PBD: 1995; Related Information: Is Part Of Proceedings of the fifth (1995) international offshore and polar engineering conference. Volume 3; Chung, J.S. [ed.] [Colorado School of Mines, Golden, CO (United States)]; Maeda, Hisaaki [ed.] [Univ. of Tokyo (Japan)]; Kim, C.H. [ed.] [Texas A and M Univ., College Station, TX (United States)]; PB: 753 p.
- Country of Publication:
- United States
- Language:
- English
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